ause the wind
pushes the kite or sail aside." It will, however, be worth while to look
for a more scientific answer. The kite cannot travel in the direction of
the wind because it is confined by a string. But the face is so attached
to the string that it inclines at an angle to the direction of the wind.
Now, when a force meets an inclined surface which it cannot carry along
with it, but which is free to travel in another direction, the force may
be regarded as resolving itself into _two_ forces, coming from each side
of the original line. These are called the _component_ forces.

[Illustration: FIG. 169.]

To explain this we give a simple sketch of a kite in the act of flying
(Fig. 169). The wind is blowing in the direction of the solid arrow A.
The oblique surface of the kite resolves its force into the two
components indicated by the dotted arrows B and C. Of these C only has
lifting power to overcome the force of gravity. The kite assumes a
position in which force C and gravity counterbalance one another.

[Illustration: FIG. 170.]

A boat sailing across the wind is acted on in a similar manner (Fig.
170). The wind strikes the sail obliquely, and would thrust it to
leeward were it not for the opposition of the water. The force A is
resolved into forces B and C, of which C propels the boat on the line of
its axis. The boat can be made to sail even "up" the wind, her head
being brought round until a point is reached at which the force B on the
boat, masts, etc., overcomes the force C. The capability of a boat for
sailing up wind depends on her "lines" and the amount of surface she
offers to the wind.

THE BALLOON

is a pear-shaped bag--usually made of silk--filled with some gas lighter
than air. The tendency of a heavier medium to displace a lighter drives
the gas upwards, and with it the bag and the wicker-work car attached to
a network encasing the bag. The tapering neck at the lower end is open,
to permit the free escape of gas as the atmospheric pressure outside
diminishes with increasing elevation. At the top of the bag is a wooden
valve opening inwards, which can be drawn down by a rope passing up to
it through the neck whenever the aeronaut wishes to let gas escape for a
descent. He is able to cause a very rapid escape by pulling another cord
depending from a "ripping piece" near the top of the bag. In case of
emergency this is torn away bodily, leaving a large hole. The ballast
(usually sand) carried